In situ grazing on plankton <10 μm by the boreal sponge Mycale lingua

Abstract

Ultraplankton, heterotrophic and autotrophic plankton <5 μm, are the most abundant food source in the world's oceans, yet their role as a food source for macroinvertebrates is largely unexamined. We quantified in situ feeding on heterotrophic and autotrophic plankton < 10 μm by the boreal sponge Mycale lingua using measurements that quantified sponge feeding efficiencies, pumping rates, and abundance to determine the contribution of plankton <10 μm to sponge carbon intake. Using dual-beam flow cytometry we identified 5 populations of plankton <10 μm: heterotrophic bacteria, Prochlorococcus, Synechococcus-type cyanobacteria, autotrophic eucaryotes <3 μm, and autotrophic eucaryotes 3 to 10 μm. Mycale lingua nonselectively grazed on all types of plankton < 10 μm. Prochlorococcus was filtered with the highest efficiency (93 %), followed by Synechococcus-type cyanobacteria (89%), autotrophic eucaryotes 3 to 10 μm (86 %), heterotrophic bacteria (74 %), and autotrophic eucaryotes <3 μm (72%). We conservatively estimate that M. lingua at naturally occurring densities can obtain 29 mg C d-1 m-2 feeding on plankton < 10 μm, with 74 % resulting from ultraplankton, suggesting that ultraplankton are an important overlooked component of benthic-pelagic coupling.